With the (relatively) recent publication of Guilds of Ravnica, it occurred to me that the format of each guild entry is well suited to each of the factions, particularly in the DM section. Each entry goes something like:
- Guild allies
- 1d20 plot hooks
- Guild adversaries
- 1d20 enemies
- Adventure location (not headquarters)
- Guild as a campaign adversary
That format could be tweaked a little bit. Maybe have the main list of plot hooks be specific to Sigil, with an additional set for the planes.
Frankly, the Guild already can be ported over to Sigil with little difficulty. Azorius Senate for the Hardheads, Boros Guild for the Mercykillers, Izzet League for Xaositects, etc.